Conveyance with electronic control for left and right motors

ABSTRACT

A conveyance (10) is provided with: automatic limiting (223) of the rate of change in power supplied to left (26a or 232a) and right propulsion motors, whether electrically or hydraulically propelled; dynamic braking of electric propulsion motors (26a and 26b) that is achieved by shorting a motor winding (150a) during a portion (233a) of an interval (217a) between power pulses (207a) of a pulse-width-modulated driving voltage (209a); power-off braking that is achieved by shorting a motor winding (150a) when no power pulses are being supplied; extended life of relays (144a &amp; 148a) that is achieved by preventing current flow to the motors during opening and closing of the relay contacts (154a and 156a); and a solid-state switching device (168a and 170a) that is controlled by a signal in a single conductor (200a) and that provides an effective delay (219a and 221a) in switching between two circuits (152a and 160a).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to controlling devices for electric orfluid actuators, and to conveyances propelled by electric or fluidactuators. More particularly, the present invention relates to acontrolling device that provides accuracy in the control of speed anddirection of electrically or hydraulically propelled vehicles, thatlimits the rate of change in the difference of rotational speed of leftand right propulsion motors, and that provides dynamic braking forelectric motors.

2. Description of the Prior Art

Conveyances of various types, for transporting people, for materialhandling, and for propelling self-propelled propelled machinery, haverequirements for extremely high maneuverability.

One way to obtain extremely high maneuverability is to separately andvariably control the speed and direction of rotation of left and rightwheels or other traction elements. When the traction elements are movingat the same speed, but in opposite directions, the conveyance will pivotaround in a fixed location, giving the ultimate in maneuverability.

In the design of some larger conveyances, such as bulldozers, it hasbeen customary to use a system of clutches and brakes to control boththe direction of rotation and the speed of endless tracks. While the useof clutches and brakes do provide extremely high maneuverability,including pivotal turns, the control of the direction of movement is farfrom being precise.

Systems using clutches and brakes for steering are not satisfactory foruse with vehicles that must maneuver in close quarters. For instance,small fork lift trucks that are used to unload boxcars and trucktrailers need better control of maneuverability than can be attained bythe use of clutches and brakes.

One way to attain more precise control of the speed and direction of thewheels, or other traction elements, is to convert the power intoelectric or hydraulic power, and then to utilize the controllabilitythat is available with the use of electric or hydraulic power toseparately and variably control the speed and direction of rotation ofleft and right propulsion motors.

However, achieving high maneuverability by separately controlling thevelocity and direction of rotation of the wheels, or other tractionelements, may make a conveyance difficult to control, or even dangerous.

For instance, it is desirable for some field harvesting machines, suchas swathers, to have the ability to make pivotal turns; but it would bedangerous to attempt to make a pivotal turn at full harvesting speed.

If the rate of change of speed of the individual wheels is limited, thenthe machine will be sluggish in acceleration, and may be dangerouslyslow in deceleration.

The problem of controllability is particularly acute in wheelchairs orother conveyances which are steered by separately and variablycontrolling the speed of rotation and direction of rotation of thewheels.

Typically, a separate D.C. electric motor has been drivingly connectedto left and right of the wheels of a wheelchair by chains or belts, andby friction rollers that separately engage the rubber tires of thewheels.

D.C. electric motors are capable of producing variable speeds that arein accordance with the electrical power that is delivered to them, andto the load that is imposed upon them. D.C. electric motors are alsocapable of reversible operation by reversing the electrical potentialthat is applied to the terminals.

Thus, manually actuated controls have been provided that separately andvariably supply electric power from a battery to left and rightpropulsion motors to provide changes in speed, to provide turns, toreverse the direction of movement, and to make pivotal turns by rotatingone wheel forward and the other wheel backward.

One popular type of manual control includes a control lever that ismoved forward in accordance with a desired speed forward, that is movedrearward in accordance with a desired speed in reverse, that is movedboth forward and to one side to make a turn while moving forward, thatis moved directly to one side to make a pivotal turn.

However, many who are disabled have severe hand tremors that render themunable to use electrically propelled wheelchairs.

The hand tremors have not posed a serious problem for control of forwardand reverse speeds, because hand tremors do not particularly effect theflexing action of the wrist that is used to control forward and reverseoperation. Also, friction between the forearm and the armrest of thewheelchair helps to steady the arm.

But, those with severe hand tremors have been unable to controlelectrically propelled wheelchairs because hand tremors are primarily atorsional movement of the wrist, and the torsional tremor increasestremendously as an effort is made to position the control lever.

Thus, as a person with severe hand tremors has tried to control thepositioning of the control lever, the tremor in his hand has moved thecontrol lever rapidly from one side to the other, giving signals forfirst one, and then the other motor to rotate faster, resulting in rapidturns in one direction and then the other, and resulting in such erraticmovement that he has not been able to control the wheelchair withoutbumping into other patients, furniture, doors, and walls.

Another problem that has attended prior art designs is that, even forthose who do not have hand tremors, control of speed and direction hasbeen uncertain because of the lack of dynamic braking. For instance,when the control lever has been positioned to reduce the electricalpower to the left propulsion motor and thereby turn to the left, inertiaof the wheelchair and occupant has driven the left propulsion motorthrough the drive train that connects the left propulsion motor to theleft wheel; and the wheelchair has not responded by turning as signalledby the control lever.

A third problem has been a relatively poor overall efficiency of thedrive trains that connect the electric motors to respective ones of thewheels; so that an unnecessarily large and heavy battery has beenrequired.

Typical prior art designs have been so heavy and so unwieldy totransport that the usual way of transporting them has been to load theminto a van by the use of a hydraulic lift. This has drastically reducedthe mobility of the patient, has detrimentally reduced his opportunitiesto visit away from his home or the care facility, or has resulted inunnecessarily high expense for a vehicle that will accommodate both thepatient and his wheelchair.

Even for the electric wheelchairs that have not been so unwieldy thatthey cannot be transported in a station wagon or in the trunk of asedan, the excess weight of both the wheelchair and the battery havemade it a strenuous job for friends or relatives to disassemble thewheelchair, load the wheelchair and battery separately into a car,reassemble the wheelchair at another location, and repeat the processwhen they return the patient to the care facility.

However, if prior art designs of electrically propelled wheelchairs hadused drive trains with better efficiencies, particularly betterefficiency when inertia of the conveyance and occupant is driving theelectric motor, then the problem of insufficient dynamic braking,particularly in making turns, would have been more severe.

A fourth problem has been inadequacy, or the entire lack, of automaticdynamic braking in the power-off condition.

A fifth problem has been in poor contact life of the relays that areused to reverse the potentials of the electric motors, resulting inunnecessary expense, and resulting in loss of use of the wheelchair forextended periods of time while spare parts are being obtained and neededrepairs are being made.

There are thousands of incapacitated people who would be able to gain agreater degree of self reliance, and some would be able to become a partof the work force of their country if they were able to control sometype of self-propelled conveyance.

Thus the present invention can help handicapped people gain a bettersense of dignity and self-worth, and to help many of them becomeproductive members of society.

SUMMARY OF THE INVENTION

The present invention provides a power driven conveyance in which powerto left and right propulsion motors is separately and variablycontrolled in response to a manually positioned control, similar to thetype used with computer games.

The control lever is oriented with relation to the conveyance so thatmoving the control lever forward results in maximum power in the forwarddirection being delivered to both the left and right propulsion motors.

In like manner, maximum power in the rearward direction is delivered toboth motors when the control lever is moved directly rearward, maximumpower is delivered to the left and right motors in opposite directionsand pivotal turns are achieved when the control lever is moved directlyto one side or the other, and various percentages of power in forwardand reverse directions are provided when the control lever is positionedat various distances from the neutral position in various directions.

Manual positioning of the control lever separately and variably actuatesthe wiper arms of left-propulsion and right-propulsion potentiometers.Each of the potentiometers provides two variable resistances, one fromthe arm to one leg thereof, and another from the arm to the other legthereof.

The following description will describe operation for only one of themotor drives, since both sides function the same, and both clarity andbrevity are best achieved in this manner.

The right-propulsion potentiometer cooperates with a signal supplyvoltage of eight volts that is applied across its legs and functions asa voltage divider to provide a right-propulsion signal.

The right-propulsion signal is supplied as the input to two operationalamplifiers. When the right-propulsion signal is more than four volts,one of the operational amplifiers provides a forward-rotation signal forcontrolling the right propulsion motor; and when the right-propulsionsignal is less than four volts, the other of the operational amplifiersprovides a reverse-rotation signal for controlling the same propulsionmotor.

A forward-propulsion comparator receives the forward-rotation signal andcooperates with a first power transistor to actuate a forward-polarityrelay. In like manner, a reverse-propulsion comparator receives thereverse-propulsion signal and cooperates with a second power transistorto actuate a reverse-polarity relay. The forward-polarity andreverse-polarity relays control the polarity of the driving voltage thatis supplied to the right propulsion motor, and thus control thedirection of rotation of the right propulsion motor.

But, the actual supplying of electrical power, and the varying of theelectrical power that is supplied, is controlled by separate means whichfunctions as follows.

The system uses two diodes to receive the forward-rotation signal andthe reverse-rotation signal, and to develop a power-control signal. Thepower-control signal varies from zero to four volts when an attenuationcontrol is adjusted to allow maximum speed and power; and thepower-control signal is attenuated to lower maximum voltages when lowermaximum acceleration, speed, and power are desired.

A sawtooth generator and the power control signal cooperate with acomparator to develop a pulse-width-modulated control signal whose pulsewidths are proportional to the magnitude of the power-control signal.

The same sawtooth generator also cooperates with a comparator in theleft-propulsion circuitry to develop a pulse-width modulated controlcircuit that cooperates with other components for driving theleft-propulsion motor.

The pulse-width-modulated control signal cooperates with a transistor toprovide a pulse and brake signal. The pulse and brake signal ispulse-width modulated as is the pulse-width control signal, but isamplified in power.

The pulse and brake signal controls two field-effect transistors. Thefirst field-effect transistor receives the pulse and brake signal andpulses a connection to ground, so that the supply voltage is pulsed tothe right-propulsion motor, thereby supplying a pulse-width, modulateddriving voltage to the right propulsion motor. The width of the pulsesdetermines the effective driving voltage.

It should be remembered that the polarity of the supply voltage that isapplied to the right propulsion motor has been determined by theforward-rotation and reverse-rotation relays, and the first field-effecttransistor determines the width of the pulses of the supply voltage thatare applied to the right propulsion motor.

The second field-effect transistor cooperates with the pulse and brakesignal to short the motor winding of the right propulsion motor duringat least a portion of the no-power intervals that separate the voltagepulses of the pulse-width-modulated driving voltage.

This shorting of the motor windings during a portion of the no-powerintervals between pulses causes the right propulsion motor to operate asan electrically loaded generator, and to provide dynamic braking.

However, if the motor winding were shorted for even a small portion ofthe time when pulses of the driving voltage were being applied to themotor winding, severe damage would be done to the circuit components.Thus, a delay circuit is provided that prevents this occurrence.

The delay circuit includes diodes, resistors, and the parasiticcapacitance of the field-effect transistors, and provides atime-interval between the end of one pulse of the pulse-width-modulateddriving voltage and shorting of the motor winding.

The delay circuit also provides a time-interval between the cessation ofshorting the motor winding and the start of the next pulse of theeffective driving voltage.

The present invention includes means for limiting the rate of change inthe difference of power that is delivered to the left-propulsion andright-propulsion motors, while leaving the change in the rates of powerthat can be delivered substantially unaffected when the rates of changeof power to both motors are generally equal.

In the preferred configuration, a capacitor which is connected acrossthe arms of the two potentiometers, limits the change in the controlvoltages when the control lever is positioned in a manner that changesthe resistances of the left-propulsion and right-propulsionpotentiometers differently, and one of the propulsion signals tries tochange more rapidly than the other propulsion signal. But, when thecontrol lever is positioned to equally increase or decrease the power toboth motors, the voltages of the right and left propulsion signalschange equally and the capacitor does not see a difference indifferential voltage; therefore, the differential limiting circuit doesnot affect acceleration or deceleration when changes in electrical powerare substantially equal to both propulsion motors.

This limiting of the rate of the difference in power delivered to thetwo motors provides a conveyance that can be controlled by people havingsevere hand tremors; because the spurious signals produced by handtremoring are time-averaged.

In addition to dynamic braking and differential change limiting, thepresent invention provides extended relay life and dynamic braking whenno pulses of power are being supplied to the motor.

The field-effect transistors cooperate with the relays to pulse thepower after the relays are closed, and to cease delivering power beforethe relays open, thereby avoiding arcing across the relay contacts, andthereby resulting in greatly extended service life for the relays.

In their unenergized state, the relays short the motor winding, therebyachieving power-off dynamic braking even when the battery is removedfrom the conveyance.

In summary, the present invention provides a conveyance, a motor drive,and a control, in which: dynamic braking is provided by shorting themotor winding during a portion of the intervals between power pulses;differential control limiting provides ease and accuracy of control bylimiting the rate of change in the difference of power that can besupplied to one motor with respect to the other motor; power-off dynamicbraking is achieved by shorting the motor winding when no power pulsesare being supplied to the motors; and extended relay life is achieved bypreventing the relay contacts from making or breaking under load.

Differential control limiting is applicable to both electric and fluidmotors; dynamic braking is applicable to any electric motor that isdriven by voltage pulses whether width-modulated, amplitude-modulated,or unmodulated; power-off dynamic braking is applicable to various uses,particularly with permanent magnet motors; and the circuitry forincreasing relay life is particularly applicable to reversible electricmotors that are driven by pulsed driving voltages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation of a conventional wheelchair to which theelectric motor drive of the present invention has been added;

FIG. 2 is a front elevation of the electric wheelchair of FIG. 1, takensubstantially as shown by view line 2--2 of FIG. 1;

FIG. 3 is a perspective view of the drive unit of the electricwheelchair of FIG. 1, taken at a perspective angle that is upward andrearward from that of FIG. 1;

FIG. 4 is a simplified representation of a prior art control lever whichprovides X and Y axis resistances proportional to movement of thecontrol lever;

FIG. 5 is a schematic drawing of the source of electrical power, and theregulated voltages, for the systems of FIGS. 9 and 10;

FIG. 6 is a schematic drawing of the sawtooth generator that is a partof pulse-width modulation of the driving voltages for both motors;

FIG. 7 shows the capacitor that limits the difference in change in powerthat is supplied to the left and right propulsion motors;

FIG. 8A illustrates the wave form of the pulse-width-modulated drivingvoltage and shows the effective delay at the start of each voltagepulse;

FIG. 8B illustrates the wave form of the dynamic braking pulses andshows the effective delay at the start of each pulse;

FIG. 8C shows the wave forms of FIGS. 8A and 8B superimposed;

FIGS. 9A and 9B combine to provide a schematic drawing of the motor andelectronics for driving the right wheel, and may be considered ascombining to form FIG. 9;

FIG. 10 is a schematic drawing of the motor and electronics for drivingthe left wheel;

FIG. 11 is a schematic diagram showing a variation of the embodiment ofFIGS. 6-9 in which the electronic control, and the differential controllimiting thereof, is used to control hydraulic motors; and

FIG. 12 is a schematic drawing showing a variation of the FIGS. 9 and 10embodiment in which field-effect transistors are used to control boththe polarity of the power and the application of the pulses, therebyeliminating the need for mechanical relays.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, and more particularly to FIGS. 1 and 2,an electric wheelchair 10 includes large wheels, or propulsion elements,12a and 12b that are attached to a frame 14, a seat bottom 16 and a seatback 18 that are both attached to the frame 14, castor wheels 19a and19b that are attached to the frame 14, and footrests 20a and 20b thatare attached to the frame 14. The wheelchair discussed thus far istypical of the prior art.

The electric wheelchair 10 includes a drive unit, 22a, and another driveunit 12b, which is generally a mirror image of the drive unit 22a.

Referring now to FIG. 3, the drive unit 22a includes a mounting plate24a which is secured to the frame 14 of the wheelchair 10, a firstelectric motor, or right electric motor, 26a of the permanentfield-magnet type which is mounted to the mounting plate 24a and whichincludes a motor shaft 28a that extends through the mounting plate 24a,a small pulley, or drive pulley, 30a which is mounted onto the motorshaft 28a, a stationary spindle 32a which is attached to the mountingplate 24a and which extends orthogonally outward therefrom, a largepulley, or driven pulley, 34a which is rotatably mounted onto thestationary spindle 32a, a drive roller 36a which is fixedly secured tothe large pulley 34a and which drivingly engages a tire 38a of the wheel12a, and two belts 40a which preferably are O-rings of a syntheticelastomer, and which engage grooves 42a of the small pulley 30a andgrooves 44a of the large pulley 34a.

A roller guard 46a is disposed circumferentially around the drive roller36a, is fixedly attached to the stationary spindle 32a, and includes alongitudinal slot 48a which allows the drive roller 36a to engage thetire 38a.

As seen in FIG. 2, a second motor, or left motor, 26b is at a differentheight than the right motor 26a; so that the motors, 26a and 26b, bypasseach other when the wheelchair 10 is folded with the wheels 12a and 12bproximal to one another.

As seen in FIG. 1, the wheelchair 10 includes a control box 50 which isattached to the frame 14 close to an armrest 52a of the electricwheelchair 10, and which includes a control lever 54, an ON-Off switch56, and a speed-power limiting control 58.

Referring now to FIG. 5, a twelve volt battery, or source of electricalpower, 60 is connected to a voltage regulator 62 by the ON-OFF switch 56of FIG. 1. The voltage regulator 62 includes output conductors 64, 66,and 68 which provide potentials of one, four, and eight volts,respectively, to parts of the circuits of FIGS. 9 to 12 which utilizerespective ones of these three different potentials.

A conductor 70, which is connected to the switch 56, furnishes voltageto parts of the circuits of FIGS. 6 and 9 to 12 which indicate apotential of twelve volts.

Referring now to FIGS. 9A and 9B, FIGS. 9A and 9B combine to provide acircuitry for the right motor 26a that hereafter will be referred to asFIG. 9.

Referring now to FIGS. 9 and 10, the circuitry of these two figures isthe same; and a sawtooth generator 72 of FIG. 6 provides a sawtoothsignal in a conductor 74 for use with the circuitry of both FIGS. 9 and10.

FIG. 9 includes a first potentiometer, or right motor potentiometer,76a; and FIG. 10 includes a second potentiometer, or left motorpotentiometer 76b.

An X-Y control, or manual control 78, of one of the types that arecommercially available, is shown in pictorial form in FIG. 4. The manualcontrol 78 includes the control lever 54 of FIGS. 1 and 2, thepotentiometers 76a and 76b of FIGS. 9 and 10, arcuate levers 80a and80b, and an actuating lever 82.

In the prior art type of manual control as depicted by FIG. 4, thecontrol lever 54 of FIGS. 1 amd 2 is mounted for pivotal movement abouta point 84 by the use of a spherical surface, not shown, so that theactuating lever 82 pivots about the point 84 in exactly the oppositedirection as the control lever 54, and for distances that are in a fixedrelationship to movement of the control lever 54.

The arcuate levers 80a and 80b are mounted for rotation about axes 85aand 85b by shafts 86a and 86b, are coupled to respective ones of thepotentiometers 76a and 76b, include actuating slots 87a and 87b whichreceive the actuating lever 82, and depend semicircularly downwardbetween the shafts 86a and 86b; so that movement of the control lever 54in X or Y directions, or in any combination thereof, produces arotational movement of the potentiometer 76a proportional to movement ofthe control lever 54 in the X direction, and rotational movement of thepotentiometer 76b proportional to movement of the control lever 54 inthe Y direction.

Moving the control lever 54 to positions 88a-88d, which representcorners of an allowable range of movement indicated by a square 89,results in full forward power for both electric motors, 26a and 26b,when the control lever 54 is positioned at the position 88a, fullreverse for both electric motors when the control lever 54 is positionedat the position 88b, a right pivotal turn with the left motor 26b atfull forward and the right motor 26a at full reverse when the controllever 54 is in the 88c position, and a left pivotal turn when thecontrol lever 54 is in the 88d position.

Since the circuitries of FIGS. 9 and 10 are identical, only FIG. 9,which includes the circuitry which drives the right electric motor 26a,will be described.

Referring now to FIG. 9, an electric motor drive, or right wheel drive,90a includes the potentiometer 76a. The potentiometer 76a includes anupper leg 92a that is connected to the conductor 68 of FIG. 5 andreceives a potential of eight volts therefrom, a lower leg 94a that isconnected to ground, as shown, and a wiper arm 96a.

Movement of the wiper arm 96a above a midpoint in the resistance of thepotentiometer 76a produces a forward-rotation signal that increases fromfour to eight volts in a conductor 98a as the wiper arm 98a movesupwardly from the midpoint in the resistance; and movement of the wiperarm 96a below the midpoint in the resistance of the potentiometer 76aproduces a reverse-rotation signal that varies from four volts down tozero as the wiper arm 96a moves downwardly from the midpoint.

The forward-rotation signal in the conductor 98a is supplied to an inputresistor 100a and to an input resistor 102a. The input resistor 100a isconnected to the negative input terminal of an operational amplifier104a; and the input resistor 102a is connected to the positive inputterminal of an operational amplifier 106a.

The positive input terminal of the operational amplifier 104a isconnected to four volts by an input resistor 108a and to ground by aresistor 110a. The negative input terminal of the operational amplifier106a is connected to four volts by an input resistor 112a; and thepositive input terminal of the operatinal amplifier 106a is connected toground by a resistor 114a.

The output of the operational amplifier 104a is a forward-rotationsignal; and the output of the operational amplifier 106b is areverse-rotation signal.

The outputs of the operational amplifiers 104a and 106a are connectedtogether by means of diodes 116a and 118a; and feedback resistors 120aand 122a are connected to a junction 124a that is intermediate of thediodes 116a and 118a, and to respective ones of the negative inputterminals of the operational amplifiers, 104a and 106a.

Continuing to refer to FIG. 9, if the feedback resistor 120a wereconnected directly to the output terminal of the operational amplifier104a, rather than between the diodes, 116a and 118a, then a decreasebelow four volts in the conductor 98a, as produced by the wiper arm 96amoving downwardly from the midpoint of the resistance in thepotentiometer 76a, would produce a voltage that would vary from zero tofour volts at the output terminal of the operational amplifier 104a.

However, since the feedback resistor 120a is connected between thediodes 116a and 118a, the voltage between the diodes 116a and 118avaries from zero to four volts with movement of the wiper arm 96a fromthe midpoint of the potentiometer 76a downwardly toward zero volts; andthe voltage at the output terminal of the operational amplifier 104a ishigher by the voltage drop across the diode 116a, which is approximatelysix-tenths of a volt.

In like manner, with an increase in voltage in the conductor 98a fromfour volts to eight volts, as the wiper arm 96a is moved upwardly towardeight volts, the operational amplifier 106a produces a voltage thatincreases from zero to four volts at the junction 124a, and that isapproximately six-tenths of a volt higher at the output of theoperational amplifier 106a.

The operational amplifiers 104a and 106a cooperate with the diodes 116aand 118a to form an absolute value circuit. That is, whether the wiperarm 96a of the potentiometer 76a moves upwardly above the midpoint ofthe resistance thereof, or moves downwardly below the midpoint of theresistance thereof, a power-control signal is produced at the junction124a which increases as a function of the wiper arm 96a of thepotentiometer 76a moving away from the midpoint of the resistancethereof. The use of this power-control signal will be describedsubsequently.

The circuitry described thus far produces three control signals: theoperational amplifier 104a produces a forward-rotation signal in aconductor 126a; the operational amplifier 106a produces areverse-rotation signal in a conductor 128a; and the diodes 116a and118a cooperate with the forward-rotation signal in the conductor 126aand with the reverse-rotation signal of the conductor 128a to providethe power-control signal at the junction 124a.

The forward-rotation signal is supplied to the positive input terminalof a comparator 130a; and the reverse-rotation signal is supplied to thepositive input teminal of a comparator 132a. Both of the comparators,130a and 132a, are connected to the one volt source of FIG. 5; so thatboth comparators, 130a and 132a, have a threshold of approximately onevolt.

Continuing to refer to FIG. 9, since the outputs of the operationalamplifiers, 104a and 106a have outputs that are approximately zero voltswhen the wiper arm 96a is at the midpoint of the resistance of thepotentiometer 76a, neither will have sufficient voltage to produce anoutput from the respective one of the comparators, 130a or 132a.

When the signal voltage in the conductor 98a is a few tenths of a voltbelow the four volt input to the positive input resistors of theoperational amplifier 104a, the output of the operational amplifier 104awill exceed the one volt threshold of the comparator 130a; and when thesignal voltage in the conductor 98a is a few tenths of a volt above thefour volt input to the negative input resistors of the operationalamplifier 106a, the output of the operational amplifier 106a will exceedthe one volt threshold of the comparator 132a.

Thus, it is theoretically impossible for the comparators, 130a and 132a,to produce outputs signalling both forward and reverse rotation of theelectric motor 26a for any position of the wiper arm 96a. Instead, thewiper arm 96a must be offset four-tenths of a volt on either side of thefour volt mid-point to produce a signal that initiates rotation of theelectric motor 26a in either the forward or reverse direction.

The output terminals of the comparators 130a and 132a are connected tothe eight volt conductor 68 of FIG. 5 by pull-up resistors 134a and 136arespectively, and to forward-power transistor 138a and reversepower-transistor 140a, respectively.

The forward-power transistor 138a is connected to ground, and to twelvevolts through a relay coil 142a of a forward-power relay 144a; and thereverse-power transistor 140a is connected to ground, and to twelvevolts through a relay coil 146a of a reverse-power relay 148a.

The right electric motor 26a includes a motor winding 150a which isconnected to the relays, 144a and 148a, as shown; and the relays, 144aand 148a, are in their unenergized positions, as shown; so that themotor winding 150a is shorted by the relays, 144a and 148a, and byconductors 152a.

Thus, when neither of the relay coils, 142a or 146a, is energized, thesystem shorts the motor winding 150a, thereby causing the electric motor26a to function as an electrically loaded generator, and therebyproviding dynamic braking.

This condition of dynamic braking occurs in three different modes. Itoccurs when the manually selected position of the control lever 54 ofFIGS. 1 and 2 is such that neither relay coil, 142a or 146a, isenergized; it occurs when the switch 56 of FIG. 5 is in the OFFposition, as shown; and it occurs when the battery 60 is removed fromthe circuit.

Thus, the system provides power-off dynamic braking as well as providingdynamic braking during power-on conditions.

When the comparator 130a provides an output, the transistor 138aenergizes the relay coil 142a, and a relay contactor 154a is moved to acontact 156a, thereby connecting an end 158a of the motor winding 150ato a twelve volt conductor 160a.

In like manner, when the relay coil 146a is energized, a relay contactor162a is moved to a contact 164a, connecting an end 166a of the motorwinding 150a to the twelve volt conductor 160a.

Thus the relays 144a and 148a function to determine which of the ends,158a or 166a, of the motor winding 150a is connected to the twelve voltconductor 160a, function to determine the polarity of the power beingsupplied to the electric motor 26a, and thereby determine the directionof rotation of the electric motor 26a.

However, completion of the circuitry to supply power to the electricmotor 26a is dependent upon a field-effect transistor, or FET 168a. Thefunctioning of the field-effect transistor 168a, and anotherfield-effect transistor, or FET, 170a, will be described subsequently.

Continuing to refer to FIG. 9, it was shown previously that apower-control signal is developed at the junction 124a, whereasdirection-control signals are developed in the conductors 126a and 128a.

The power control signal of junction 124a is connected to apotentiometer 172a which is mechanically connected to a potentiometer172b of FIG. 10. The potentiometers 172a and 172b serve to attenuate thepower-control signals of the junctions 124a of FIG. 9 and 124b of FIG.10, and thus to provide an adjustable means for limiting the maximumpower supplied to the electric motors 26a and 26b.

The attenuated power-control signal in a conductor 174a is supplied tothe negative terminal of a comparator 176a; and the positive terminal ofthe comparator 176a is connected to the sawtooth generator 72 by theconductor 74.

Referring now to FIG. 6, the sawtooth generator 72 includes anoperational amplifier 178, resistors 180, 182, 184, 186, and 188, adiode 190, and a capacitor 192. The sawtooth generator 72 is a standardrelaxation circuit and a detailed description can be found in bothelectronic textbooks and handbooks. Thus, it is sufficient to note thata sawtooth voltage is delivered to the conductor 74 that varies from aminimum of one volt to a maximum of three volts.

Referring again to FIG. 9, the comparator 176a, together with a pull-upresistor 194a which is connected between the output of the comparator176a and the twelve volt conductor 160a, produces an output in aconductor 196a whenever the input to the negative terminal of thecomparator 176a is greater than the sawtooth voltage which the sawtoothgenerator 72 supplies to the positive input terminal of the comparator176a via the conductor 74.

The result is that a pulse-width-modulated control signal is produced inthe conductor 196a whose pulse widths are a function of the magnitude ofthe attenuated power-control signal in the conductor 174a.

The conductor 196a is connected to a transistor 198a. The transistor198a is connected to the twelve volt source in the conductor 160a by aconductor 200a and a pull-up resistor 202a, and is connected to ground.

The field-effect transistor 168a is an N channel enhancement mode MOSFETwhich turns on when its gate 204a is increased above ground potential;and the field-effect transistor 170a is a P channel enhancement modeMOSFET which turns on when its gate 206a is decreased below its sourcepotential.

When the output of the comparator 176a is producing a voltage pulse, thegate 204a of the FET 168a is above ground potential; and the FET 168acompletes the circuitry of the electric motor 26a by connecting one ofthe ends, 158a or 166a, of the motor winding 150a to ground.

Of course, the one of the ends, 158a or 166a, that is connected toground by the FET 168a depends upon the positions of the relaycontactors 154a and 162a of the relays 144a and 148a.

But when the comparator 176a is not producing an output in the conductor196a, as is the condition between pulses of the pulse-width-modulatedcontrol voltage, then the voltage in the conductor 196a is approximately0.7 volts and current flow in the pull-up resistor 202a, and the voltagedrop thereof, brings the voltage on the gate 206a of the FET 170a downbelow the source voltage of the twelve volt conductor 160a.

With the voltage on the gate 206a below the source voltage, the FET 170aconducts, connecting the twelve volt conductor 160a to the conductors152a. Since one of the ends, 158a or 166a, of the motor winding 150a isconnected to the twelve volt conductor 160a by one of the relaycontactors, 154a or 162a, the result is that both ends, 158a and 166a,of the motor winding 150a are connected to the twelve volt conductor160a; the motor winding 150a is shorted; the electric motor 26afunctions as an electrically loaded generator; and the electric motor26a provides dynamic braking.

Referring now to FIGS. 8A, 8B, 8C, and 9, the FET 168a pulses aconnection to ground so that driving-voltage pulses, or power pulses,207a of the supply voltage are applied to the electric motor 26a thatare an effective driving voltage, or pulse-width-modulated drivingvoltage, 209a whose pulse widths 211a are generally proportional tomanual positioning of the control lever 54 and the potentiometer 76a.

The FET 170a provides dynamic braking pulses 213a whose pulse widths215a are interposed between adjacent ones of the power pulses 207a thatare supplied by the FET 168a.

When the pulse widths 207a of the driving voltage 209a become wider,no-power intervals, or no-voltage intervals, 217a between pulses becomesmaller, and the dynamic braking is reduced; and as the pulse widths207a of the driving voltage 209a become narrower, the no-power intervals217a between the voltage pulses 209a become wider, and the dynamicbraking is increased.

Therefore, the dynamic braking has little effect on the efficiency ofthe drive when the electric motor 26a is operating at, or near, maximumpower. But the dynamic braking is quite effective in providing thedeceleration that is needed to provide controllability, particularly thedeceleration that is required to make turns with a conveyance which ispropelled by two electric motors that separately and variably controlleft and right wheels.

Continuing to refer to FIG. 9, it has been shown that both the FET 168aand the FET 170a are controlled by the voltage in the conductor 196a.Remember that the FET 168a applies power to the electric motor 26a, andthe FET 170a shorts the motor winding 150a; so it is apparent that theFETS, 168a and 170a, control functions that must not occur at the sametime.

The present invention includes means for providing an effective delay219a in starting each braking pulse 213a subsequent to the end ofrespective ones of the power pulses 207a.

Also, the present invention includes means for providing an effectivedelay 221a in starting each power pulse 207a subsequent to the end ofrespective ones of the braking pulses 213a.

The means for providing the effective delays, 219a and 221a, include acoupling resistor 208a, diodes 210a and 212a, time-delay resistors 214aand 216a, and parasitic capacitors, 218a and 220a, of the FETS, 168a and170a, which are indicated by dash-lines.

The output of the transistor 198a is delivered to the gate 204a of theFET 168a by means of the coupling resistor 208a and the time-delayresistor 214a. Now any transistor that has appreciable current-carryingcapacity has some parasitic capacitance, as indicated by the parasiticcapacitors 218a and 220a. So, an increase in voltage at the gate 204a,in response to an increase in voltage in the conductor 200a at theoutput of the transistor 198a, is delayed by current flowing through thetime-delay resistor 214a to charge the parasitic capacitor 218a.

However, when the voltage falls at the output of the transistor 198a,the parasitic capacitor 218a is discharged rapidly through the diode210a which bypasses the time-delay resistor 214a.

Thus, the time-delay resistor 214a, the parasitic capacitor 218a and thediode 210a cooperate to provide the effective delay 221a in the start ofa pulse 207a of driving voltage 209a; and these same elements cooperateto promptly shut off the FET 168a.

In like manner, the output of the transistor 198a is delivered to thegate 206a of the FET 170a by means of the coupling resistor 208a and thetime-delay resistor 216a.

When the output of the transistor 198a decreases, a decrease in thevoltage at the gate 206a, below the supply voltage in the conductor160a, is delayed by current flowing through the time-delay resistor 216aas the parasitic capacitor 220a of the FET 170a discharges.

However, when the voltage increases at the output of the transistor198a, the parasitic capacitor 220a is charged rapidly through the diode212a which bypasses the time-delay resistor 216a.

Thus, the time-delay resistor 216a, the parasitic capacitor 220a, andthe diode 212a cooperate to delay a decrease in voltage on the gate206a, and to provide the effective delay 219a in each dynamic brakingpulse 213a subsequent to cessation of a power pulse 207a; and these sameelements cooperate to promptly increase the voltage on the gate 206a,and promptly shut off the FET 170a.

So, means is provided for effectively delaying the start of shorting themotor winding 150a, and for delaying the start of the next power pulse207a, thereby preventing the motor winding 150a from being shortedduring the time that a pulse 207a of the driving voltage 209a is beingsupplied to the motor winding 150a.

These delays, or effective time-intervals, 219a and 221a, are achievedas a time-delay resistor, 214a or 216a, a diode 210a or 212a, and aparasitic capacitor, 218a or 220a, and cooperate to limit the rate ofchange in the magnitude of a signal applied to the FETs, or solid-statedevices, 168a or 170a. The results are that the solid-state devices,168a and 170a, are made to turn on more slowly than they turn off; andso both of the solid-state devices, 168a and 170a, can be operated byinverse changes of the magnitude of a signal in a single conductor, theconductor 200a, without danger of being instantaneously conductive.

Referring now to FIGS. 7, 9, and 10, the wiper arms 96a and 96b of thepotentiometers 76a and 76b are connected together by the conductors 98aand 98b, a time-averaging capacitor 222, and a variable resistor 224.

When the control lever 54 is positioned to make a sudden change in theposition of one of the wiper arms, 96a or 96b, with respect to formerpositions of the wiper arms, 96a and 96b, the capacitor 222 delays thechange in voltage in the conductor 98a or 98b that is connected to theone of the wiper arms, 96a or 96b, that has been repositioned abruptly.

Thus, the capacitor 222 time-averages changes in the differences of thesignal supply voltages that are being supplied by the potentiometers 76aand 76b; and the capacitor 222 functions as a change limiting means 223for limiting the rate of change in the difference in power that can besupplied to one motor, 26a or 26b, with respect to the other motor 26bor 26a.

The variable resistor 224 is an optional part of the change limitingmeans 223; but can be used to provide a means for selectively varyingthe rate of change in the difference in power that can be supplied toone motor, 26a or 26b, with respect to the other motor, 26b or 26a.While it would be possible to vary the conductance, as represented bythe capacitor 222, particularly by switching various capacitors inbetween the conductors, 98a and 98b, the variable resistor 224 providesa means for infinitely varying the limiting of the rate of change in thesignal supply voltages.

Referring now to FIG. 11, a fluid motor drive 230a is similar to theelectric motor drive 90a of FIG. 9, is used to control a reversiblefluid motor 232a, and achieves limiting of the rate of change in thedifference in the speed of the fluid motor 232a, and a similar fluidmotor, not shown, similar to that which has been described for theelectric motors 26a and 26b.

The fluid motor drive 230a is used in cooperation with another fluidmotor drive, not shown. Both of the fluid motor drives, 230a and theother fluid motor drive, are used with the sawtooth generator 72 of FIG.6, are connected thereto by the conductor 74, and cooperate with thesawtooth generator to separately provide, and to separately utilize,pulse-width-modulated control voltages.

In like manner, the fluid motor drives, 230a, and not shown, areconnected together by the change limiting means 223 of FIG. 7.

The fluid motor drive 230a includes a directional control valve 234awith a solenoid coil 236a, the fluid motor 232a which is connected tothe directional control valve 234a by motor conduits 238a and 240a, afluid pump 242a which is connected to a fluid reservoir 244a by an inletconduit 246a and to a proportional output valve 248a by an outletconduit 250a.

The fluid pump 242a and the fluid reservoir 244a provide a source offluid power, the fluid power is delivered to the proportional outputvalve 248a by the outlet conduit 250a, the proportional output valve248a delivers pressurized fluid to the directional control valve 234a bya supply conduit 252a, the directional control valve 234a delivers thepressurized fluid to the fluid motor 232a through one of the motorconduits 238a or 240a, the fluid motor 232a returns fluid to thedirectional control valve 234a through the other of the motor conduits,240a or 238a, the directional control valve 234a returns fluid to theproportional output valve 248a through a return conduit 254a, and theproportional output valve 248a returns fluid to the fluid reservoir 244athrough a reservoir conduit 256a.

Positioning the potentiometer 76a produces an output from the transistor138a as described in conjunction with FIG. 9.

The transistor 138a connects the solenoid coil 236a between twelve voltsand ground whenever the potentiometer 76a is positioned to develop avoltage in the conductor 98a that is less than four volts.

The directional control valve 234a is positioned to supply pressurizedfluid to one of the motor conduits, 238a or 240a, when the solenoid coil236a is energized, and is positioned to supply pressurized fluid to theother of the motor conduits, 240a or 238a, when the solenoid coil 236ais not energized. Therefore, only one comparator, 130a, is needed,whereas two comparators, 130a and 132a, were used with the electricmotor drive 90a of FIG. 9.

The proportional output valve 248a includes a solenoid coil 258a whichcontrols the flow rate of pressurized fluid that is delivered to thedirectional control valve 234a proportional to the effective drivingvoltage that is applied across the solenoid coil 258a. Or, alternately,the proportional output valve controls the flow of fluid coming backfrom the fluid motor 232a.

As described in conjunction with FIG. 9, a pulse-width-modulated controlsignal is developed at the output of the comparator 176a as the sawtoothoutput of the sawtooth generator 72 is compared with the attenuatedvoltage out of the potentiometer 172a. This pulse-width-modulatedcontrol signal cooperates with the FET 168a to provide apulse-width-modulated voltage as has been described in conjunction withFIG. 9; and this pulsed voltage is applied to the solenoid coil 258a ofthe proportional output valve 248a to provide a fluid flow rate that isproportional to positioning of the potentiometer 76a.

The fluid motor drive 230a includes a diode 260a which is connectedacross the solenoid coil 258a of the proportional output valve 248a, andwhich prevents excessive voltages from being applied to the output ofthe FET 168a when the magnetic field of the solenoid coil 258acollapses. In like manner, a diode 262a is placed across the solenoidcoil 236a of the directional control valve 234a to prevent excessivevoltages from being applied to the output of the transistor 138a whenthe magnetic field of the solenoid coil 236a collapses.

The electronic circuitry of the embodiment of FIG. 11 does not includedynamic braking; so the circuitry of FIG. 9 that includes the FET 170ais not needed. Consequently, time delays between pulses of drivingvoltage and pulses of braking voltage are not needed. So, only one FET,168a, is required; and the time-delay resistor 214a and the diode 210aof FIG. 9 are not needed. Thus, in FIG. 11 the coupling resistor 208a isconnected directly to the gate 204a of the FET 168a.

Hydraulic circuits and components for achieving control of fluid motors,including the direction of rotation, rotational speed, and dynamicbraking are common to the art; so the hydraulic circuitry of FIG. 11 isrepresentative of one of the many ways in which the control of directionof rotation, speed of rotation, and limitation of the rate of change oftwo fluid motors can be achieved with the present invention.

Referring now to FIG. 12, an electric motor drive 268a is provided whichis similar to the electric motor drive 90a of FIG. 9, but which has theadvantage of eliminating the mechanical relays, 144a and 148a, of FIG.9.

The electric motor drive 268a cooperates with an identical electricmotor drive, not shown, with the sawtooth generator 72 of FIG. 6, andwith the change limiting means 223 of FIG. 7 for limiting the rate ofchange in the difference of power supplied to two electric motors, asdescribed in conjunction with FIG. 9.

The electric motor drive 268a of FIG. 12 includes the operationalamplifiers 104a and 106a of FIG. 9. The operational amplifiers 104a and106a are connected to the potentiometer 76a, to the four volt source,and to ground by identically numbered and identically named parts asthose of FIG. 9. The embodiment of FIG. 12 uses feedback resistors 270aand 272a to feed back the outputs of the amplifiers, 104a and 106a, totheir respective inputs.

When the control lever 54 of FIGS. 1 and 2 is in its centered position,the wiper arm 96a of the potentiometer 76a is at the midpoint of theresistance, and the wiper arm 96a delivers four volts to the inputs ofboth of the operational amplifiers, 104a and 106a.

Since the amplifiers, 104a and 106a, are differential amplifiers withfour volts on both inputs, the outputs of both amplifiers, 104a and106a, are at ground potential when the wiper arm 96a supplies four voltsto both operational amplifiers, 104a and 106a.

As the wiper arm 96a is moved downwardly, the output of the amplifier104a in creases above ground potential; and as the wiper arm 96a ismoved upwardly, the output of the amplifier 106a increases above groundpotential. Notice that the amplifiers, 104a and 106a, cannot produceoutputs simultaneously.

If, for instance, the amplifier 104a is producing an output, this outputis compared to the sawtooth waveform of the sawtooth generator 72 by acomparator 274a and a pulse-width-modulated control signal is developedin a conductor 276a by the comparator 274a and a pull-up resistor 278athat is connected between the conductor 276a and the twelve volt source.

The pulse-width-modulated control signal is delivered to a transistor290a by the conductor 276a. The pulse-width-modulated control signal isinverted and the level of the signal is shifted by the transistor 290aand by a pull-up resistor 292a which is connected between the transistor290a and the twelve volt source.

The output of the transistor 290a is connected to FETS 294a and 296a.The FET 294a is an N channel enhancement mode MOSFET which turns on whenits gate 298a is increased above ground potential; and the FET 296a is aP channel enhancement mode MOSFET which turns on when its gate 300a isdecreased below its source potential.

When the wiper arm 96a of the potentiometer 76a is below the mid-pointof the resistance, and the voltage on the wiper arm 96a is less thanfour volts, the output of the comparator 274a is high during the pulsewhich is developed by the comparator 274a, the output of the transistor290a is low, the voltage to the gate 300a of the FET 296a is below thesource voltage, the FET 296a is on, and a terminal 301a of the motor 26ais connected to the twelve volt source by the FET 296a.

In like manner, the output of a comparator 302a is connected to FETS304a and 306a. THE FET 304a is an N channel enhancement mode MOSFETwhich turns on when its gate 308a is increased above ground potential;and the FET 306a is a P channel enhancement mode MOSFET which turns onwhen its gate 310a is decreased below its source potential.

Continuing the description of operation with the wiper arm 96a of thepotentiometer 76a below the mid-point in its resistance, in thiscondition, there is no output from the comparator 302a, the output of atransistor 312a is high, the voltage to the gate 310a of the FET 306a ishigh, the FET 306a is off, the gate 308a of the FET 304a is high, theFET 304a is on, and the FET 304a connects a terminal 314a of theelectric motor 26a to ground.

Thus, in the condition described, the FETS 296a and 304a cooperate todetermine the direction of rotation of the electric motor 26a by makingconnections respectively to the terminals 301a and 314a of the electricmotor 26a; and the FET 296a connects the electric motor to the twelvevolt source with pulse widths that are proportional to the positioningof the wiper arm 96a below the mid-point of the potentiometer 76a toprovide a pulse-width-modulated driving voltage.

Continuing to describe the operation of the FIG. 12 embodiment, with avoltage on the wiper arm 96a that is less than four volts, thecomparator 274a is supplying pulses of voltage that arepulse-width-modulated. However, between voltage pulses of the comparator274a, the output of the comparator 274a is low, the output of thetransistor 290a is high, the gate 300a of the FET 296a is at sourcevoltage, the FET 296a is off, the gate 298a of the FET 294a is high, theFET 294a is on, and the FET 294a is connecting the terminal 301a of theelectric motor 26a to ground.

So, between voltage pulses of the comparator 274a, the terminal 301a ofthe electric motor 26a is connected to ground by the FET 294a and theterminal 314a of the electric motor 26a is connected to ground by theFET 304a.

Therefore, the circuitry that has been described provides dynamicbraking between pulses of the pulse-width-driving voltage by shortingthe electric motor 26a, and thereby causing the electric motor 26a tofunction as an electrically loaded generator.

Operation of the circuitry with a voltage of more than four volts on thewiper arm 96a functions in like manner as has been described forvoltages of less than four volts on the wiper arm 96a, the differencebeing that the comparator 302a cooperates with the FETS 306a and 304a toprovide connections that determine the direction of rotation of theelectric motor 26a, that pulse the power, and that provide dynamicbraking, and the circuitry to ground is completed by the FET 294a.

The output of the transistor 290a is coupled to the FETS 294a and 296aby circuitry that includes a coupling resistor 316a; and the output ofthe transistor 312a is coupled to the FETS 304a and 306a by circuitrythat includes a coupling resistor 318a.

An increase in voltage at the gate 298a of the FET 294a, and a delay inturning on the FET 294a, is achieved by a diode 320a, a time-delayresistor 322a, and a parasitic capacitor 324a which is inherent in thedesign of the FET 294a, in the manner that has been described inconjunction with FIG. 9.

Thus, the period 331a of 0.008 seconds is divided among: the interval,or delay, 221a before starting the driving voltage pulse 207a, thedriving voltage pulse width 211a, the interval, or delay 219a beforestarting the braking pulse 213a, and the braking pulse width 215a.

While the driving voltage pulse widths 211a may be varied from zero tothe full period 331a, an example of the inverse modulation of the pulsewidths 211a and 215a is as follows: if the driving voltage pulse widths211a are varied from 7.5 percent (0.0006 seconds duration) to 87.5percent (0.007 seconds duration) of the period 331a, since the sum ofthe intervals 219a and 221a is 5 percent (0.0002 seconds each), thebraking pulse widths 215a will vary from 87.5 percent (0.007 secondsduration) to 7.5 percent (8.0006 seconds duration).

In like manner, a decrease in voltage at the gate 300a of the FET 296abelow the source voltage, and a delay in turning on the FET 296a, isachieved by a diode 326a, a time-delay resistor 328a, and a parasiticcapacitor 330a which is inherent in the design of the FET 296a.

The construction thus described provides a time-interval between thecessation of one voltage pulse of the pulse-width-modulated drivingvoltage and an adjacent one of the dynamic braking pulses, and atime-interval between the end of one braking pulse and the start of thenext voltage pulse.

Thus, it can be seen that the electric motor drive of FIG. 12 providesthe same advantages as the electric motor drive 90a of FIG. 9, and alsoeliminates the necessity of using mechanical relays, such as the relays144a and 148a.

Referring again to FIGS. 8A, 8B, and 8C, the power pulses 207a are at anamplitude of the source voltage, which preferably is either 12 or 24volts. The frequency of the sawtooth which is generated by the sawtoothgenerator 72 preferably is 125 hertz; so a period 331a of one completecycle preferably is 0.008 seconds.

In a typical design the effective delays, 219a and 221a, areapproximately 200 microseconds; and the amplitude of the braking pulses213a is about one volt.

The braking pulse 213a is applied for a portion 233a of the no-powerinterval 217a; and the portion 233a is less than the no-power interval217a by the effective delay 219a of the brake pulse 213a.

As can be clearly seen in the preceding description, the motor 26a isisolated from the source 60 during the no-power intervals 217a. Thus,rather than attempting to return the braking energy to the source ofelectrical power 60 by regenerative braking, the braking energy of thepresent invention is dissipated in the motor 26a and in the circuitwhich includes the FET 170a and the motor winding 150a, and, generally,the energy of braking is dissipated in heat.

For purposes of understanding the appended claims, a first electricembodiment of the invention includes a motor control 332a which includesall of the components of the motor drive 90a of FIG. 9 except for thepotentiometer 76a and 76b and the electric motor 26a, and includes thesawtooth generator 72 of FIG. 6.

A second electric embodiment includes a motor control 334a includes thesawtooth generator 72 of FIG. 6, and includes all of the components ofthe motor drive 268a of FIG. 12 except for the potentiometer 76a and theelectric motor 26a.

In the hydraulic embodiment, a motor control 336a includes the sawtoothgenerator 72 of FIG. 6, and includes all of the components of the motordrive 230a of FIG. 11 except for the potentiometer 76a and the fluidmotor 232a.

While the motors 26a and 232a have been shown as rotary motors, it willbe apparent that the present invention will provide differential controllimiting for changes in linear velocity of linear motors as well.Therefore, the word "motor" is to be construed in its broader sense ofan actuator which is either rotary or linear.

The hydraulic embodiment of FIG. 11 includes a source of fluid power338a which includes the pump 242a and the reservoir 244a.

The motor control 332a of FIGS. 6 and 9 includes both a power control,or driving voltage control, 340a and an electronic control 342a. Thepower control 340a includes the transistors 138a and 140a, thetransistor 198a, the FET 168a, and the relays 144a and 148a.

The electronic control 342a of FIGS. 6 and 9 includes the amplifiers104a and 106a, the comparators 130a and 132a, the potentiometer 172a,the comparator 176a, and the sawtooth generator 72.

The motor control 334a of FIGS. 6 and 12 includes both a power control,or driving voltage control, 344a, and an electronic control 346a.

In like manner, the motor control 336a of FIGS. 6 and 11 includes both apower control 348a and electronic control 350a.

The power control 344a of FIG. 12 includes the transistors 290a and 312aand the FETS 296a and 304a; and the power control 348a of FIG. 11includes the transistor 138a, the transistor 140a, the FET 168a, thedirectional control valve 234a, and the proportional output valve 248a.

The manual control 78 includes the potentiometers 76a and 76b, thecontrol lever 54, and any mechanism that interconnects the control lever54 to the potentiometers 76a and 76b, such as the prior art mechanism ofFIG. 4.

The embodiment of FIG. 9 includes a motor loading means 354a whichincludes the FET 170a, and the embodiment of FIG. 12 includes a motorloading means 356a, which includes the FET 294.

The sawtooth generator 72 of FIG. 6 cooperates with the comparator 176aof FIG. 9 to provide a pulse-width modulator. In like manner, thesawtooth generator 72 cooperates with the comparator 274a of FIG. 12 toprovide a pulse-width modulator.

In summary, the present invention provides apparatus and method forproviding a conveyance, a motor drive, and a control, in which: dynamicbraking is provided by shorting the motor winding during a portion ofthe interval between power pulses; differential control limiting isprovided that assures ease and accuracy of manual control by limitingthe rate of change in the difference of power that can be supplied toone motor with respect to the other motor; power-off braking is achievedby shorting the motor winding when no power pulses are being supplied tothe motors; extended relay life is achieved by transmitting power to themotors only when the relay contacts are closed; and a solid-stateswitching device is provided that is controlled by a signal in a singleconductor and that provides an effective delay in switching.

Differential control limiting is applicable to both electric and fluidmotors; dynamic braking is applicable to any electric motor that canfunction as an electrically loaded generator and that is driven byvoltage pulses whether width-modulated, amplitude-modulated, orunmodulated; power-off braking is applicable to various uses,particularly with reversing motors; and the circuitry for increasingrelay life is particularly applicable to reversible electric motors.

While specific apparatus and parameters have been disclosed in thepreceding description, and while numbers of specific parts that havebeen described in the specification have been included in the claims, itshould be understood that these specifics have been given for thepurpose of disclosing the principles of the present invention and thatmany variations thereof will become apparent to those who are versed inthe art. Therefore, the scope of the present invention is to bedetermined by the appended claims and the recitations thereof.

INDUSTRIAL APPLICABILITY

The present invention is applicable to conveyances in which left andright traction elements are separately and variably controlled by leftand right electric or fluid motors, is applicable to conveyances inwhich the operator has hand tremors, and is applicable to conveyances inwhich dynamic braking of electric motors is needed.

What is claimed is:
 1. An electric motor drive (90a or 268a) havingdynamic braking, which motor drive comprises:an electric motor (26a);motor control means (332a or 334a), being connected to said electricmotor (26a), for supplying an effective driving voltage (209a) to saidmotor that comprises voltage pulses (207a) interspersed with intervals(217a) of substantially no power; said motor control means comprisesmeans for varying said effective driving voltage; and motor-loadingmeans (354a or 356a), being operatively connected to said electricmotor, for placing an electrical load on said electric motor during aportion (233a) of a plurality of said no-power intervals while saidmotor control means continues to supply said voltage pulses to saidmotor; whereby said electric motor provides dynamic braking during saidportions of said plurality of said no-power intervals.
 2. An electricmotor drive (90a or 268a) as claimed in claim 1 in which said motorcontrol means (332a or 334a) comprises a pulse-width-modulator(72a+176a, or 72+274a);said driving voltage pulses comprisepulse-width-modulated pulses (207a) of a substantially constant voltage;and said means for varying said effective driving voltage comprisesmeans (104a) for selectively varying the pulse widths (211a) of saidvoltage pulses (207a).
 3. An electric motor drive (90a or 268a) asclaimed in claim 28 in which said electric motor (26a) includes a motorwinding (150a) having first (158a) and second (166a) ends; andsaidmotor-loading means (354a or 356a) comprises means (170a or 294a) forproviding an electrical flow path between said ends of said motorwinding during said portions (233a) of said plurality of no-powerintervals (217a).
 4. An electric motor drive (90a or 268a) as claimed inclaim 1 in which said electric motor (26a) includes a motor winding(150a) having first (158a) and second (166a) ends; andsaid motor-loadingmeans (354a or 356a) comprises means for effectively interconnectingsaid ends of said motor winding during said portions of said pluralityof no-power intervals.
 5. An electric motor drive (90a or 268a) asclaimed in claim 1 in which said motor control means (332a or 334a)comprises means for effectively providing an interval (219a or 221a)between one of said electrically loaded portions (233a) of said no-powerintervals (217a) and an adjacent one of said voltage pulses (207a);saidmotor control means (332a or 334a) comprises a first solid-state device(168a); said motor loading means (354a or 356a) comprises a secondsolid-state device (170a or 294a); and said means for effectivelyproviding an interval comprises means (214a or 216a) for turning on oneof said solid-state devices more slowly than said one solid-state deviceturns off.
 6. An electric motor drive (90a or 268a) as claimed in claim1 in which said motor control means (332a or 334a) comprises means foreffectively providing an interval (219a or 221a) between one of saidelectrically loaded portions (233a) of said no-power intervals (217a)and an adjacent one of said voltage pulses (207a);said motor controlmeans (332a or 334a) comprises a first solid-state device (168a); saidmotor loading means (354a or 356a) comprises a second solid-state device(170a or 294a); and said means for effectively providing an intervalcomprises means (210a and 214a, or 212a and 216a) for causing one ofsaid solid-state devices to turn on more slowly than the other of saidsolid-state devices turns off.
 7. An electric motor drive (90a or 268a)as claimed in claim 1 in which said electric motor (26a) includes amotor winding (150a) having first (158a) and second (166a) ends;saidmotor control means (332a or 334a) comprises a first solid-state device(168a); said motor-loading means (354a or 356a) comprises means,including a second solid-state device (170a or 294a) for providing anelectrical flow path between said ends of said motor winding during saidportions of said plurality of no-power intervals; said motor controlmeans (332a or 334a) comprises means for effectively providing aninterval (219a or 221a) between one of said electrically-loaded portions(233a) and an adjacent one of said voltage pulses (207a); and said meansfor effectively providing an interval comprises means (214a) for turningon one of said solid-state devices more slowly in response to a changein the magnitude of a signal than the other of said solid-state devicesturns off in response to said change.
 8. An electric motor drive (90a or268a) as claimed in claim 1 in which said electric motor (26a) includesa motor winding (150a) having first (158a) and second (166a) ends;saidmotor control means (332a or 334a) comprises a first solid-state device(168a); said motor-loading means (354a or 356a) comprise a secondsolid-state device (170a or 294a) that effectively interconnects saidends of said motor winding during said portions (233a) of said pluralityof no-power intervals (217a); said electric motor drive furthercomprises means for effectively providing a time-interval (219a or 221a)between the interconnecting of said ends of said motor winding and anadjacent one of said voltage pulses (207a); one (168a or 170a) of saidsolid-state devices turns on with a change in the magnitude of a signalin one direction and turns off with a change in said magnitude in theother direction; and said means for effectively providing saidtime-interval comprises means for reducing the rate of change of themagnitude of said signal in said one direction.
 9. An electric motordrive (90a or 268a) as claimed in claim 1 in which said motor controlmeans (332a or 334a) includes first solid-state means (168a) for saidsupplying of said voltage pulses to said electric motor;said electricmotor (26a) includes a motor winding (150a) having first (158a) andsecond (166a) ends; said motor-loading means (354a or 356a) comprisessecond solid-state means (170a or 294a) for effectively interconnectingsaid ends of said motor winding during said portions of said no-powerintervals; said electric motor drive comprises means, comprising aresistor (216a or 322a) and a diode (212a or 322a), for effectivelydelaying said effective interconnecting of said ends of said motorwinding subsequent to respective ones of said portions of said no-powerintervals; and said electric motor drive comprises means for effectivelydelaying said voltage pulses (207a) subsequent to cessation of adjacentones of said electrically loaded portions.
 10. An electric motor drive(90a or 268a) as claimed in claim 1 in which said motor control means(332a or 334a) comprises first field-effect transistor means (168a or296a) for said supplying of voltage pulses (207a) to said electric motor(26a);said electric motor includes a motor winding (150a) having first(158a) and second (166a) ends; said motor-loading means (354a or 356a)comprises second field-effect transistor means (130a or 294a), having aparasitic capacitance (220a or 324a), for effectively interconnectingsaid ends of said motor winding during said portions (233a) of saidno-power intervals (217a); and said electric motor drive comprisesmeans, comprising a resistor (216a or 322a), a diode (212a or 322a), andsaid parasitic capacitance, for effectively delaying (219a) saidinterconnecting of said ends of said motor winding subsequent tocessation of an adjacent one of said voltage pulses (207a).
 11. Anelectric motor drive (90a) as claimed in claim 1 in which said electricmotor (26a) includes a motor winding (150a) having first (158a) andsecond (166a) ends;said motor control means (332a) comprises a powermode wherein said motor control means is connected to a source (60) ofelectrical power and a no-speed mode wherein said motor control means isisolated from said source of electrical power; said electric motor driveincludes means (144a & 148a), being operatively connected to respectiveones of said ends of said motor winding, for selectively determining thedirection of rotation of said electric motor; and said electric motordrive includes means, comprising effectively interconnecting said firstand second ends of said motor winding when said motor control means isin said no-speed mode, for making said electric motor function as anelectrically loaded generator when said motor control means is in saidno-speed mode; whereby said electric motor provides dynamic braking whensaid motor control means is in said no-speed mode.
 12. An electric motordrive (90a) as claimed in claim 1 in which said electric motor (26a)includes a motor winding (150a) having first (158a) and second (166a)ends;said motor control means (334a) comprises a power mode wherein saidmotor control means is connected to a source of electrical power (60)and a no-speed mode wherein said motor control means is siolated fromsaid source of electrical power; said electric motor drive includesrelay means (144a & 148a), comprising first (154a) and second (162a)contactors that are connected to respective ones of said ends of saidmotor winding, for selectively determining the direction of saidelectric motor; and said electric motor drive includes means (144a &148a) for providing an electrical flow path between said first andsecond ends of said motor winding when said motor control means is insaid no-speed mode; whereby said electric motor functions as anelectrically loaded generator and provides dynamic braking when saidmotor control means is in said no-speed mode.
 13. An electric motordrive (90a) as claimed in claim 1 in which said electric motor driveincludes relay means (144a & 148a), comprising first (154a and 156a) andsecond (162a and 164a) pairs of electrical contacts, for makingelectrical connections that determine the direction of rotation of saidelectric motor 26A;said motor control means (332a) comprises a powermode wherein said voltage pulses (207a) are applied to said electricmotor and a no-speed mode wherein said voltage pulses are not applied tosaid electric motor; and said electric motor drive includes means forclosing one of said pairs of electrical contacts before one of saidvoltage pulses is supplied to said motor when said motor control meansgoes from said no-speed mode to said power mode, and for opening saidone pair of said electrical contacts after cessation of said supplyingof said one voltage pulse when said motor control means goes from saidpower mode to said no-speed mode; whereby arcing between said one pairof electrical contacts is obviated and the service life thereof isextended.
 14. An electric motor drive (90a or 268a) as claimed in claim1 in which said electric motor drive includes a second (26b or 26a)electric motor;said electric motor drive includes a second motor controlmeans, being connected to said second electric motor, for supplying asecond effective driving voltage to said second electric motor thatcomprises second voltage pulses that are interspersed with secondno-power intervals; said electric motor drive includes means (352) forseparately and selectively varying said voltage pulses that are suppliedto the first and second electric motors; and said electric motor driveincludes means (223), being operatively connected to the first motorcontrol means and to said second motor control means, for limiting therate of change in the difference of voltage pulses supplied to one ofsaid electric motors with respect to the other of said electric motorswhile permitting relatively unrestricted rates of changes in voltagepulses supplied to said electric motors when said changes aresubstantially equal to both of said electric motors.
 15. An electricmotor drive (90a or 268a) as claimed in claim 1 in which said electricmotor drive includes a second electric motor (26b);said electric motordrive includes second driving voltage control means, being connected tosaid second electric motor, for supplying a second effective drivingvoltage to said second electric motor that comprises second voltagepulses interspersed with second intervals of substantially no voltage;said electric motor drive includes means (104a) for separately andselectively varying said effective driving voltage that is supplied tosaid first electric motor; said second driving voltage control meansincludes means for separately and selectively varying said effectivedriving voltage that is supplied to said second electric motor; and saidelectric motor drive includes means (223), comprising a capacitor (222),being operatively connected to the first motor control means and to thesecond motor control means, for limiting the rate of change in thedifference of the effective driving voltage supplied to one of saidelectric motors with respect to the other of said electric motors whilepermitting relatively unrestricted rates of changes in said effectivedriving voltages supplied to said first and second electric motors whensaid changes in said effective driving voltages are substantially equalto both of said electric motors.
 16. An electric motor drive (268a) asclaimed in claim 1 in which said electric motor (26a) includes a winding(150a) having first (158a) and second (166a) ends;said electric motor isreversible by reversing the polarity of said voltage pulses (207a); andsaid electric motor drive includes solid-state means (296a & 304a) formaking connections to said first and second ends of said motor winding,and for reversing the polarity of said voltage pulses supplied to saidelectric motor.
 17. An electric motor drive (268a) as claimed in claim 1in which said electric motor (26a) includes a winding (150a) havingfirst (158a) and second (166a) ends;said electric motor is reversible byreversing the polarity of said voltage pulses; said electric motor driveincludes solid-state means, comprising a solid-state device (294a or296a) with a parasitic capacitance (324a or 330a), for makingconnections to said first and second ends of said motor winding, andmeans for reversing the polarity of said voltage pulses supplied to saidelectric motor; and said motor control means 334a includes means,comprising said parasitic capacitance, for effectively providing a timeinterval (219a or 221a) between one of said electrically loaded portions(213a) and an adjacent one of said voltage pulses (207a).
 18. Anelectric motor drive (90a or 268a) as claimed in claim 1 in which one ofsaid portions (233a) of said no-power intervals (217a) has atime-duration of between 0.0006 and 0.007 seconds.
 19. An electric motordrive (90a or 268a) as claimed in claim 1 in which said electrical loadis of the type in which said electrical load is generally converted intoheat.
 20. A conveyance (10) having dynamic braking, which conveyancecomprises:a propulsion element (12a or 12b); an electric motor (26a or26b) being operatively connected to said propulsion element; motorcontrol means (332a or 334a), being connected to said electric motor,for supplying an effective driving voltage (209a) to said electric motorthat comprises a plurality of voltage pulses (207a) to said electricmotor that are interspersed with no-power intervals (217a); andmotor-loading means (354a or 356a), being operatively connected to saidelectric motor, for placing an electrical load (213a) on said electricmotor during a portion (233a) of a plurality of said no-power intervalswhile said motor control means continues to supply said voltage pulsesto said motor; whereby said electric motor provides dynamic brakingduring said portions of said no-power intervals.
 21. A conveyance (10)as claimed in claim 20 in which said motor control means (332a or 334a)comprises means (104a) for inversely modulating said pulses (207a) ofdriving voltage (209a) and said electrical load (213a).
 22. A conveyance(10) as claimed in claim 20 in which said motor control means (332a or334a) comprises means (104a) for modulating said voltage pulses (207a)over a first range of pulse widths (215a) and modulating said electricalload (213a) over a second range of pulse widths (215a); andone of saidranges of pulse widths includes one pulse width that is between 0.0006and 0.007 seconds.
 23. A conveyance (10) as claimed in claim 20 in whichsaid electric motor (26a) includes a motor winding (150a) having first(158a) and second (166a) ends; andsaid motor-loading means (354a or356a) comprises means (170a or 294a) for providing an electrical flowpath between said ends of said motor winding.
 24. A conveyance (10) asclaimed in claim 20 in which said conveyance includes means foreffectively providing time intervals (219a and 221a) between one of saidvoltage pulses (207a) and an adjacent one of said electrical load(213a);said placing of said electrical load (213a) on said electricmotor (26a or 26b) comprises applying a plurality of dynamic brakingpulses (213a) to said motor during said portions (233a) of saidplurality of said no-power intervals (217a); one of said plurality ofpulses (207a or 213a) is initiated by a change in the magnitude of asignal in one direction and is terminated by a change in the magnitudeof said signal in the other direction; and said means for effectivelyproviding said time interval comprises means for reducing the rate ofchange of the magnitude of said signal in said one direction.
 25. Aconveyance (10) as claimed in claim 20 in which said motor control means(332a or 334a) includes means, comprising first and second electricalcontacts (154a and 156a), for making electrical connections thatdetermine the direction of rotation of said electric motor (26a);saidmotor control means (332a or 334a) includes means for closing saidelectrical contacts before said effective driving voltage is supplied tosaid motor; and said motor control means (332a or 334a) includes meansfor opening said electrical contacts after ceasing to supply saideffective driving voltage to said motor; whereby arcing between saidelectrical contacts is obviated and the service life thereof isextended.
 26. A conveyance (10) as claimed in claim 20 in which saidconveyance includes a second motor control means (332a or 334a) thatsupplies a second effective driving voltage to a second electric motor(26b);said conveyance includes means (352), being operatively connectedto the first motor control means and to said second motor control means,for separately and selectively varying said effective driving voltages;and said conveyance includes means (223), being operatively connected tothe first said motor control means and to said second motor controlmeans, for limiting the rate of change in the difference of said drivingvoltages while permitting larger rates of changes in said drivingvoltages when said changes in said effective driving voltages aresubstantially equal.
 27. A conveyance (10) as claimed in claim 20 inwhich said voltage pulses (207a) are supplied by a source of electricalpower (60); andsaid motor (26a) is generally isolated from said sourceof electrical power during said no-power intervals.
 28. A conveyance(10) as claimed in claim 20 in which said placing of said electricalload on said motor (26a) comprises applying dynamic braking pulses(213a) to said motor; andone of said pulses (207a or 213a) has atime-duration of between 0.0006 and 0.007 seconds.
 29. A motor drive(90a or 268a) of the type which includes an electric motor (26a), theimprovement which comprises:motor control means (332a or 334a), beingoperatively connected to said electric motor, for supplying an effectivedriving voltage (209a) to said motor that comprises a plurality ofdriving voltage pulses (207a); and dynamic braking means (354a or 356a),being operatively connected to said motor, for applying dynamic brakingpulses (213a) to said motor that are interspersed between respectiveones of said driving voltage pulses while said motor control meanscontinues to supply said voltage pulses to said motor; whereby saiddynamic braking pulses provide dynamic braking for said motor drive. 30.A motor drive (90a) as claimed in claim 29 in which said motor drivecomprises means (104a) for modulating said dynamic braking pulses(213a).
 31. A motor drive (90a) as claimed in claim 29 in which saidmotor drive comprises means (104a) for modulating the width (215a) ofsaid dynamic braking pulses (213a).
 32. A motor drive (90a) as claimedin claim 29 in which said motor drive comprises means (104a) formodulating said dynamic braking pulses (213a) over a range of pulsewidths (215a); andsaid range of pulse widths includes one pulse widththat is between .0006 and .007 seconds.
 33. A motor drive (90a or 268a)as claimed in claim 29 in which said motor control means (332a or 334a)includes means for effectively providing a time interval (219a or 221a)between one of said driving voltage pulses (207a) and an adjacent one ofsaid dynamic braking pulses (213a);one of said plurality of pulses isinitiated by a change in the magnitude of a signal in one direction andis terminated by a change in the magnitude of said signal in the otherdirection; and said means for effectively providing said time intervalcomprises means for reducing the rate of change of the magnitude of saidsignal in said one direction.
 34. A motor drive (90a or 268a) as claimedin claim 29 in which said motor control means (332a or 334a) includesmeans, comprising a resistor and a diode, for effectively providing timeintervals (219a and 221a) between one of said driving voltage pulses(207a) and the adjacent ones of said dynamic braking pulses (213a). 35.A motor drive (90a or 268a) as claimed in claim 29 in which one of saidpulses (207a or 213a) has a time-duration of between 0.0006 and 0.007seconds.
 36. A motor drive (90a or 268a) as claimed in claim 29 in whichsaid pulses (207a) of said driving voltage (209a) are supplied by asource of electrical power (60); andsaid dynamic braking means (354a or356a) does not return a substantial amount of electrical power to saidsource.
 37. A method for controlling an electric motor (26a) in anelectric motor drive (90a), which method comprises the steps of:(a)supplying a driving voltage (209a) to said motor that comprises voltagepulses (207a) that are interspersed with no-power intervals (217a); and(b) placing an electrical load (213a) on said electric motor during aportion (233a) of a plurality of said no-power intervals whilecontinuing to supply said voltage pulses to said motor.
 38. A method asclaimed in claim 37 in which said method comprises selectively varyingsaid electrical load (213a).
 39. A method as claimed in claim 37 inwhich said method comprises inversely varying said driving voltage(209a) and said electrical load.
 40. A method as claimed in claim 37 inwhich said electric motor (26a) is of the type having a motor winding(150a) with first (158a) and second (166a) ends; andsaid step of placingan electrical load (213a) on said motor comprises providing anelectrical flow path between said ends of said motor winding.
 41. Amethod as claimed in claim 37 in which said method comprises delayingone ("a" or "b") of said steps subsequent to cessation of the other ofsaid steps;said delaying step comprises initiating said one step inreponse to a signal of a given magnitude; and said delaying step furthercomprises increasing the time that is required for said signal to reachsaid given magnitude.
 42. A method as claimed in claim 37 in which saidelectric motor drive having mechanical contacts, said method furthercomprises the steps of:(a) closing said mechanical contacts (154a and156a) that determine the direction of rotation of said motor (26a); (b)supplying said driving voltage (209a) to said motor; (c) discontinuingsaid supplying of said driving voltage to said motor; and (d) openingsaid mechanical contacts subsequent to said discontinuing step.
 43. Amethod as claimed in claim 37 in which said step of placing anelectrical load on said motor (26a) comprises applying a plurality ofdynamic braking pulses (213a) to said motor; andsaid method comprisesvarying the widths (215a) of said dynamic braking pulses.
 44. A methodas claimed in claim 37 in which said step of placing an electrical loadon said motor (26a) comprises applying a plurality of dynamic brakingpulses (213a) to said motor; andsaid method comprises modulating thewidth (215a) of said dynamic braking pulses to include one pulse widththat is between .0006 and .007 seconds.
 45. A method for dynamicallybraking a conveyance (10) of the type having a first electric motor(26a) that is drivingly connected to a first propulsion element (12a),which method comprises the steps of:(a) supplying a driving voltage(209a) to said motor that comprises voltage pulses (207a) that areinterspersed with no-power intervals (217a); and (b) making saidelectric motor function as an electrically loaded generator during aportion (233a) of a plurality of said no-power intervals whilecontinuing to supply said voltage pulses to said motor.
 46. A method asclaimed in claim 45 in which said method further comprises the stepsof:(a) supplying a second effective driving voltage to a second electricmotor that comprises voltage pulses that are separated by intervals; (b)selectively and separately varying the widths (211a) of said voltagepulses supplied to the first motor with respect to the widths of saidvoltage pulses supplied to said second motor; (c) limiting the rate ofchange in the difference between said widths of said voltage pulses; and(d) permitting greater changes in the rates of change of said widthswhen said rates of change are generally equal.
 47. A method as claimedin claim 45 in which said method comprises delaying one ("a" or "b") ofsaid steps subsequent to cessation of the other of said steps;saiddelaying step comprises initiating said one step in response to a signalof a given magnitude; and said delaying step further comprisesincreasing the time that is required for said signal to reach said givenmagnitude.
 48. A method as claimed in claim 45 in which said supplyingof said voltage pulses (207a) of said driving voltage (209a) comprisescommunicating a source of electrical power (60) with said motor (26a);andsaid making of said motor (26a) to function as an electrically loadedgenerator comprises generally isolating said electric motor from saidsource during said portions (233a) of said no-power intervals (217a).